Engine Cooling Fan Circuit Check

Circuit Description
The engine cooling fan circuit operates the main cooling fan and the auxiliary cooling fan. The cooling fans are controlled by the powertrain control module (PCM)/engine control module (ECM) based on inputs from the Engine Coolant Temperature (ECT) sensor and the Air Conditioning (A/C) pressure sensor. The PCM/ECM controls the low speed cooling fan operation by internally grounding the PCM/ECM connector terminal C13.This energizes the low speed cooling fan relay and operates the main cooling fan and the auxiliary cooling fan at low speed as the cooling fans are connected in a series circuit. The PCM/ECM controls the high speed cooling fan operation by internally grounding the PCM/ECM connector terminal C13 and the PCM/ECM connector terminal C12 at the same time. This energizes the low speed cooling fan relay, the high speed cooling fan relay, and the series/parallel cooling fan relay resulting in high speed fan operation as the cooling fans are now connected in a parallel circuit.

Diagnostic Aids
- If the owner complains of an overheating problem, it must be determined if the complaint is due to an actual boil over, or the engine coolant temperature gauge indicated overheating. If the engine is overheating and the cooling fans are on, the cooling system should be checked.
- If the engine fuse block fuses EF3 or EF8 become open (blown) immediately after installation, inspect for a short to ground in the wiring of the appropriate circuit. If the fuses become open (blown) when the cooling fans are to be turned on by the PCM/ECM, suspect a faulty cooling fan motor.
- The PCM/ECM will turn the cooling fans on at low speed when the coolant temperature is 199 °F (93 °C). The PCM/ECM will turn the cooling fans off when the coolant temperature is 194 °F (90 °C).
- The PCM/ECM will turn the cooling fans on at high speed when the coolant temperature is 207 °F (97 °C). The PCM/ECM will change the cooling fans from high speed to low speed when the coolant temperature is 201 °F (94 °C).
- The PCM/ECM will turn the cooling fans on at low speed when the A/C system is on. The PCM/ECM will change the cooling fans from low speed to high speed when the high side A/C pressure is 273 psi (1882 kPa) then return to low speed when the high side A/C pressure is 210 psi (1448 kPa). When the A/C system is on, the PCM/ECM will change the cooling fans from low to high speed when the coolant temperature reaches 239 °F (115 °C) then return to low speed when the coolant temperature reaches 234 °F (112 °C).
- The cooling fan circuit can be checked quickly by disconnecting the PCM/ECM connector 2 (white) and grounding the connector terminal C13.This should create low speed cooling fan operation with the ignition ON. By grounding the PCM/ECM connector terminals C13 and C12 and turning the ignition ON, high speed cooling fan operation should be achieved.

Step 1 - 4:

Step 5 - 12:

Step 13 - 24:

Step 25 - 35:

Step 37 - 42:

Step 43 - 49:

Test Description
The number(s) below refer to step(s) on the diagnostic table.
4. This step, along with Step 5, checks for the ability of the PCM/ECM to operate the cooling fans.
8. This step, along with Step 9, checks for the ability of the PCM/ECM to operate the cooling fans in response to A/C pressure readings.
16. After confirming battery voltage and the PCM/ECM supplying a ground to the coil side of the low speed cooling fan relay, by jumpering connector terminals 30 and 87 it will be determined if the relay is at fault or a wiring problem is present.
30. The replacement PCM/ECM must be reprogrammed. Refer to the latest Techline procedure for PCM/ECM reprogramming.
31. This step checks for the presence of battery voltage to the main cooling fan when the A/C is ON. If battery voltage is present and the cooling fans are not operating, the problem is in the ground side of the cooling fan circuit.
37. By directly grounding the PCM/ECM connector terminals C13 and C12, the main and auxiliary cooling fans should run at high speed.